Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. More particularly, the present invention relates to a semiconductor device having a trench for isolation of elements and a manufacturing method thereof.
In a conventional semiconductor device, it is known to use a trench isolation to isolate semiconductor elements formed on a semiconductor substrate. FIGS. 16-21 are cross-sectional views showing conventional method of manufacturing a semiconductor device. Referring to FIG. 16, a thermal oxide film 2 of about 20 nm in thickness is formed on a surface represented by (001) plane of a silicon substrate 1. Then, a silicon nitride film 3 of about 200 nm in thickness is formed on thermal oxide film 2. A resist is applied on silicon nitride film 3, and a resist pattern 4 is formed by patterning the resist by photolithography. Silicon nitride film 3 and thermal oxide film 2 are dry-etched by using resist pattern 4 as a mask. A part of silicon substrate 1 is also etched in this step to form a recess 1h. 
Referring to FIG. 17, after removing resist pattern 4, silicon substrate 1 is dry-etched by using patterned silicon nitride film 3 and thermal oxide film 2 as a mask. This forms a trench 7 of about 300 nm in depth.
Referring to FIG. 18, plasma is used to form trench 7 by etching. A surface of the trench is thermally oxidized for the purpose of removing a layer damaged by plasma and of making a corner portion 7c of trench 7 rounded to avoid an electric field concentration at corner portion 7c. This forms a thermal oxide film 5 of about 30 nm in thickness.
Referring to FIG. 19, silicon oxide film 9 of about 500 nm in thickness is formed. This silicon oxide film 9 fills trench 7.
Referring to FIG. 20, silicon oxide film 9 is removed to expose silicon nitride film 3. Silicon oxide film 9 is removed by a method such as a chemical mechanical polishing (CMP) or an etch back.
Referring to FIG. 21, after silicon oxide film 9 is wet-etched to a prescribed amount by hydrofluoric acid, silicon nitride film 3 is removed by hot phosphoric acid. This forms a shallow trench isolation (STI) 10. Thereafter, an n-type well, a p-type well, a gate oxide film, a gate electrode, a source-drain region, etc. are formed to make an element such as a transistor.
A problem caused by the conventional manufacturing method will be described below. FIG. 22 is a cross-sectional view showing a problem caused by the conventional manufacturing process. Referring to FIG. 22, thicker thermal oxide film 5 formed by thermal oxidation is more preferable to further round corner portion 7c of trench 7. Therefore, it is conceivable to thicken thermal oxide film 5 (to 60 nm, for example) by increasing an amount of thermal oxidation, as shown in FIG. 22. However, as the amount of thermal oxide film 5 is increased, an aperture width W2 of trench 7 seen from a surface side of silicon substrate 1 will become smaller than an aperture width W1 of trench 7 shown in FIG. 18. This would make the step of filling trench 7 with silicon oxide film 9 more difficult, and the filling efficiency will be decreased. Therefore, the isolation of elements cannot be ensured and a reliability of the semiconductor device will be decreased.